Insulated wires such as those used in communications cable often include flame retardant insulating materials. In communications cables, these insulated wires are often provided as twisted pairs consisting of two insulated conductors twisted about each other to form a two conductor group. The flame retardant insulating materials used with these cables allow them to be located in the plenum of buildings or in other locations where flame retardance and low smoke generation are important properties for the cable.
The flame retardant insulating materials conventionally used with insulated wires include fluorinated polymers such as fluorinated ethylenepropylene (FEP), ethylenetrifluoroethylene (ETFE), and ethylenechlorotrifluoroethylene (ECTFE). Although the fluorinated polymers used as insulation impart the necessary flame retardant properties to the plenum cable, these polymers are generally quite expensive. Therefore, it is desirable to minimize the amount of insulated material used for surrounding the conductors, as for example, by applying a relatively thin layer of the insulating material.
It is also often desired to foam the polymer insulating material. Foamed insulating materials can further minimize the quantity of polymer required while improving the electrical transmission characteristics of the resulting cable. The insulating materials are commonly foamed with a gas blowing agent such as nitrogen or carbon dioxide. However, there are problems associated with foaming the insulating polymer material with gas blowing agents. In particular, where thin insulating layers are employed, small variations in the process conditions in applying the insulating material to the conductor can result in disproportionately large changes in the characteristics of the foamed polymer. For this reason it is difficult to maintain close manufacturing tolerances for density, thickness, dielectric constant, etc. This is particularly a problem at the high temperatures used to melt the fluorinated polymers. As a result, it is difficult to provide wires having a layer of foamed fluorinated polymer insulating material with uniform or consistent properties along the length of the wire. Therefore, the electrical properties of the cable suffer.